1. Field of the Invention
The present disclosure generally relates to a transmitter, in particular to an ultra-low power transmitter applied in multi-channel frequency shift keying (FSK) communication.
2. Description of the Related Art
Recently, the wireless communication systems have become more and more popularized in the application to the data transmission. It uses the transmitter to modulate the data to a specific frequency and transmit to the receiver. Afterwards, the receiver demodulates the received data. However, because the specific frequencies are mostly in the high-frequency band, the frequency synthesizer used in the conventional transmitters consumes high power and is not feasible in low power applications. Without a frequency synthesizer, the transmitter is only feasible for the single-channel communication or the coarse multi-channel communication.
Generally, the conventional transmitter architectures are classified with the following three types.
1. The transmitter with mixer and phase-lock loop (PLL). This transmitter modulates the data to a specific frequency, which has been processed by the frequency synthesizer through the mixer, wherein the data is to be transmitted. Then, the data would be transmitted by the power amplifier. Here, the frequency synthesizer, which is applied in this transmitter, is designed with PLL. Thereby, the entire architecture costs high power consumption because this transmitter must use the PLL.
2. The transmitter with closed-loop filter. This transmitter directly modulates the data, which are processed by the delta-sigma modulator at first, to a specific frequency at the frequency synthesizer. Then, the data would be transmitted by the power amplifier. Because the transmitter directly modulates the data to the specific frequency at the frequency synthesizer, the closed-loop filter must be existed. Thereby, the data rate would be limited when using this type of transmitter.
3. The transmitter with injection-locked oscillator. This transmitter modulates the data to different frequencies by directly amplifying the frequency to the desired band and adjusting the capacitance value of the capacitor of the voltage controlled oscillator (VCO). Because the modulation is performed by adjusting the capacitance value, the resolution of the frequency selection would be lower.